WASHINGTON — Republican senators warned Trump administration officials Tuesday that they were prepared to block the president’s effort to impose tariffs on Mexican imports, promising what would be GOP lawmakers’ most brazen defiance of the president since he took office.

During a closed-door lunch, at least a half-dozen senators spoke in opposition to the tariffs, while no one spoke in support, according to multiple people present who spoke on the condition of anonymity to discuss the private meeting.

Senators told officials from the White House and Department of Justice that there could be a disapproval vote if Trump moves forward — and this time, unlike with an earlier disapproval resolution, opponents of Trump’s tariffs could have enough support to override a veto.

“I think the administration ought to be concerned about another vote of disapproval on another national emergency act, this time trying to implement tariffs. Tariffs are not real popular in the Republican conference,” said Sen. Ron Johnson, R-Wis., summarizing what he said he had told the administration officials. “This is a different vote.”

“There is not much support in my conference for tariffs, that’s for sure,” said Majority Leader Mitch McConnell, R-Ky. He said senators hope negotiations with Mexico will be “fruitful” and the tariffs won’t happen.

The lunch meeting occurred just hours after Trump, during a news conference in London, reiterated his intention to impose the tariffs next week and said it would be “foolish” for Republican senators to try to stop him. The 5% tariffs on all Mexican goods, rising to 25% over time, are aimed at trying to force Mexico to take action to stop the tide of Central American migrants seeking entry into the United States.

The administration lawyers who attended the meeting were not clear on whether the White House would use the existing national emergency Trump declared earlier this year on the border to justify the tariffs — or issue a new declaration, senators said.

But some Republicans said the opposition to Trump’s proposed levies on Mexico runs so deep that enough GOP senators would be willing to defy the president on any disapproval vote involving the tariffs.

“I sure do,” Sen Kevin Cramer, R-N.D., said when asked whether he thought there would be at least 20 Republican votes to reject Trump’s tariffs on Mexico.

The GOP rejection votes would constitute a veto-proof margin in combination with Democratic opposition. It takes two-thirds support in the House and the Senate to overturn a presidential veto.

“I didn’t hear many senators if any say that they wanted a tariff,” Sen. Mike Braun, R-Ind., said.

Earlier, at a news conference in London, Trump said talks with Mexico will continue even as he goes forward with the tariffs.

“It’s more likely the tariffs go on and we’ll probably be talking during the time the tariffs are on,” Trump said at the news conference in Britain with Prime Minister Theresa May.

White House officials on Tuesday were preparing to implement the tariffs next week. Secretary of State Mike Pompeo and other top administration officials plan to meet with Mexican leaders Wednesday, but White House officials have been careful not to specify precise actions they expect Mexico to take in exchange for not imposing the tariffs. Some Trump advisers think that the president likes negotiating from a position of strength and that he believes he will have more leverage over Mexico if the tariffs are in position.

As for GOP talk of voting on a disapproval resolution to block the levies, Trump said, “Oh, I don’t think they will do that, I think if they did it’s foolish.”

Trump shocked U.S. lawmakers and Mexican leaders last week by announcing that he would impose a 5% tariff on all goods imported from Mexico on June 10, and then increase the levies each month if the border with the United States isn’t closed to migrants.

GOP lawmakers warned White House officials that the tariffs could imperil the chances of passing an overhaul of the 1994 North American Free Trade Agreement, but Trump has remained undeterred.

Trump suggested Tuesday that Mexico could take action to stave off the tariffs, and Mexican officials expressed optimism that they could reach a deal to avoid the penalties.

“Mexico shouldn’t allow millions of people to try and enter our country, and they could stop it very quickly and I think they will,” Trump said. “And if they won’t, we’re going to put tariffs on.”

Trump’s tone as he addressed reporters in London contrasted with that of Mexican Foreign Minister Marcelo Ebrard, who said at a news conference in Washington that he thought his country had an 80% chance of reaching a deal.

Mexico has begun a vigorous offensive to avert the U.S. tariffs.

Delegations of Mexican lawmakers and business leaders are heading to Washington to warn against the tariffs. The penalties could severely affect Mexico, which sends 80% of its exports to the United States, its top trading partner.

President Andrés Manuel López Obrador also expressed optimism Tuesday at a news conference in Mexico that the two sides “will reach an accord before the 10th of June.”

U.S. authorities have detained more than 100,000 migrants along the Mexican border in each of the past two months. Mexico has nearly tripled monthly deportations since the start of the year but is struggling to cope with the rising flow of migrants.